The invention relates generally to transparent nano-composite materials and their production. More specifically, the invention relates to the production and use of a transparent, nano-particle reinforced ceramic, such as structures suitable for use in windows for optically guided armaments.
Many modern armaments use optical guidance systems to acquire and intercept their targets. Such systems often use an optical tracking system based on receiving energy in the mid-infrared region, at wavelengths around three to four micrometers. Examples of such systems include air-to-air, heat-seeking guided missiles, and smart munitions such as guided bombs that track a target by laser back scatter. In all of these devices, it is necessary to protect the relatively fragile optical tracking systems from heat and impact events. Protection of the optics requires the use of a highly impact-resistant window that is suitably transparent in the mid infrared region.
One of the best materials for protective windows and domes is α-alumina in its single crystal form, known as sapphire. Sapphire is transparent to electromagnetic radiation having a wavelength up to 4 μm, and is extremely impact resistant at ambient temperature. Synthetic sapphire crystals can be grown in cylindrical ingots of many inches in diameter, and then cut and machined to shape for the final application. However, sapphire exhibits significantly lower strength as temperature increases, especially above about 600° C., limiting its use in hypersonic applications. Furthermore, the machining process necessary to form a final shape from a sapphire ingot is difficult to implement at a reasonable cost.
Other window materials exist, including magnesium fluoride, zinc sulfide, gallium phosphate, yttria, aluminum oxynitride and spinel. All of these materials can be processed to near net shape, but may have limitations in the combination of properties required for missile applications. For example, magnesium fluoride, zinc sulfide, and gallium phosphate may be more suitable for IR windows used at wavelengths of 10-12 microns, while yttria, aluminum oxynitride and spinel may work better for windows used at IR wavelengths of 3-5 microns. Other issues with these materials may include such problems as sensitivity to water droplet impact, which can cause the window to shatter at hypersonic speeds. Reducing grain size in single phase materials may provide some incremental enhancements in mechanical properties. However, a window or dome material must provide breakthroughs in strength, thermal shock resistance, and processability to allow projectiles to perform at hypersonic speeds.
Therefore it is desirable that a new material be identified for use in manufacturing protective windows that are transparent in the mid-IR frequencies. This material should have enhanced strength and processability over single crystal sapphire. There is also a need for processes for reliable and relatively economic manufacture of structures from such materials.